During the last few years, a substantial effort has been devoted to production of thin paperboard laminated sheet material incorporating a food engaging plastic film having a vacuum deposition layer of metal, such as elemental aluminum, which metallized layer forms a microwave interactive layer. This sheet stock is referred to as a susceptor and is produced in large quantities for the food industry. When the susceptor sheet material is wrapped around a food item, such as a pizza, or is formed into a container for encircling a food item, microwave energy can penetrate the metallized layer of the susceptor sheet material causing eddy current to flow in the metallized layer to increase the temperature of the metallized layer and, thus, the lamination sheet material itself. Consequently, the lamination sheet material is a heat source for cooking food items adjacent the susceptor material or within a container formed by the susceptor material. The susceptor material has the characteristics of a thin paperboard and can be die cut, formed and glued into various shapes. This sheet material is widely employed in the food industry and is primarily applicable to heating of crust type food items, such as the crust of a pizza or the crust of a pot pie.
By allowing the susceptor sheet material to be an independent heat source activated by microwave energy, the susceptor sheet material can obtain a relatively high temperature such as over 400.degree. F. which will heat the adjacent food item either by radiation or contact conduction. In this latter instance, with the temperature of the susceptor sheet increasing to a temperature substantially higher than the temperature possible by microwave heating of the food item directly, the susceptor sheet can cause crisping and browning of the crust of the food item. Consequently, one of the primary objectives of a microwave susceptor sheet stock is the ability to reach a relatively high temperature, which is a function of the amount of vacuum deposited aluminum forming the microwave interactive layer and the applied microwave energy. As the metal applied to the plastic film of the metallized surface increases, the obtainable temperature of the susceptor sheet will first rise and then drastically decrease. This decrease is caused by a complete covering of the plastic film. If the metallized aluminum layer is a continuous layer of aluminum, microwave energy will be reflected from the layer and will create a minor amount of eddy current heating in the metallized layer. Thus, the metallized aluminum must have a relatively thin thickness allowing passage of microwave energy to create heating of the interactive metal layer.
The thickness of the metallized layer controls the ultimate temperture reached during exposure of the susceptor sheet to microwave energy. Since the aluminum is vacuum deposited upon a thin film of plastic material, the temperature to which the interactive aluminum layer can increase is limited by the thermal characteristics of the thin plastic film onto which the interactive metallized layer is vacuum deposited. To obtain the necessary temperature for rapid and effective crisping and browning of crust material, polyethylene film has generally been replaced by polyester film. In addition, the thickness of the plastic film is generally greater than 1.0 mils to withstand the temperature to which the interactive layer is elevated during the cooking process. It has been found that even with a relatively thick layer (5 mils) of PET (polyethylene terephthalate) the temperature to which the interactive layer could be elevated during the cooking process was seriously limited. When using the more standard PET material, which is readily available in thin film, a sufficient amount of deposited aluminum on the PET film to create the desired cooking temperature caused crazing of the film during the cooking process. The temperature of the sheet would exceed 400.degree. F. and the PET would craze at about 400.degree. F. To prevent such crazing, the temperature of the interactive material was reduced by limiting the amount of metallization on the surface of the film. This, in turn, reduces the effectiveness of the susceptor sheet material for heating the food adjacent the susceptor sheet.
In view of this situation, there has been a substantial demand for a thin film material which would not craze at temperatures desired for effective cooking of food by the lamination microwave susceptor sheet material. It was suggested that laminated high temperature plastic material could be used for supporting the metallized microwave interactive layer. Higher temperature plastic materials were not available in thin film. Less than 5 mils is an acceptable film thickness for the purposes of use in a microwave susceptor sheet of the type including a thin plastic film which is metallized and supported on a paperboard. Further, even at the high range of acceptable thickness, high temperature plastic was too expensive for susceptor sheet.
Efforts to increase the thickness of the plastic film for the purposes of withstanding higher temperatures has proven ineffective. The increased thickness resulted in an insulation layer between the interactive metallized layer and the food being cooked. In Quick U.S. Pat. No. 4,713,510 the crazing problem was solved by placing the food against the paperboard instead of against the plastic film. Thus, crazing of the film which supported the metallized microwaveable interactive material was irrelevant. This solution was not effective because this paperboard created even a greater insulation barrier between the interactive layer and the food item being cooked by the microwave energy.
In accordance with the present invention, PCTA copolyester plastic film sold by Eastman Chemical Products a subsidiary of Eastman Kodak Company under the No. 6761 has proven extremely advantageous. This material has a melting point of 545.degree. F. with an inherent viscosity 0.96. The crystalline peak melting point is 545.degree. F. The temperature of crystallization on cooling is 375.degree. F. Glass transition temperature is 208.degree. F. This material is sold under the trademark THERMX and is not available in thin film. In accordance with the present invention, this plastic material is to be extruded into a sheet having a thickness of less than 1.0 mils. Directly onto this material vacuum deposited aluminum is to be added with a metallization thickness sufficient to raise the temperature above over 400.degree. F., and preferably over 450.degree. F. PCTA copolyester is a polymer of cyclohexanedimethanol and terephthalic acid with another acid substituted for a portion of the terephthalic acid.
In accordance with the present invention, there is provided a laminated susceptor sheet for use in a disposable container adapted to heat a quantity of food in the container when exposed to microwave energy. The lamination comprises a microwave interactive layer of electrically conductive metal having a thickness which is sufficiently small to cause the microwave interactive layer, when subjected to microwave energy, to heat up to a temperature of over 400.degree. F., which temperature is sufficient to heat the surface of the food in heat transfer relationship with the susceptor material. Of course, between the interactive layer and the food there is provided a protective thin plastic film having sufficient stability at high temperature that it will not degrade when the lamination is subjected to microwave energy to heat the surface of the quantity of food. In accordance with the present invention, the protective plastic film onto which the interactive layer of electrically connective metal is deposited, is PCTA copolyester film having a thickness of less than about 1.0 mil with a melting point of over 500.degree. F. The protective plastic film with the interactive layer of electrically conductive metal is supported onto a paper stock material having a sufficient structural stability at high temperature necessary for heating the surface of the quantity of food to maintain its physical shape at such high temperatures. The interactive metallized layer of conductive metal is formed onto the plastic film in accordance with standard practice of vacuum depositing the microwaveable interactive layer onto the plastic film after which the film is bonded directly to the paper stock material in a manner to cause the film and interactive layer to be held in bonded relationship with the paper stock.
In accordance with another aspect of the invention, there is provided a method of making a laminated susceptor sheet for use in a disposable container adapted to heat a quantity of food. This method includes extruding a thin plastic film of less than about 1.0 mils from PCTA copolyester plastic, vacuum depositing a layer of elemental aluminum onto the plastic film with aluminum having a thickness which is sufficiently small to cause the aluminum layer, when subjected to microwave energy, to heat to the desired temperature of over 400.degree. F. and bonding the plastic film directly onto one side of a paperboard stock material.
The primary object of the present invention is the provision of a unique microwave susceptor sheet formed by laminating a thin plastic film, onto which there is deposited a metallized layer, onto a paperboard sheet, which susceptor sheet does not craze at high temperatures and can be heated to a temperature exceeding about 450.degree. F. without crazing or degrading of the plastic film against which the food being cooked is placed.
Another object of the present invention is the provision of a heat susceptor sheet, as defined above, which sheet is relatively inexpensive to manufacture, can be employed for browing and crisping crust material at relatively high temperatures and can employ normal microwave susceptor technology.
These and other objects and advantages will become apparent from the description of the preferred embodiment.